Preheat oven to 325F/170C/Gas Mark 3. Grease an 8 round baking pan and line with parchment paper. Set aside.

Melt chocolate and butter in a double boiler. Remove from heat and transfer to a large bowl. In a small cup, mix espresso powder, water, vanilla and salt; stir into chocolate. Set mixture aside to cool. With an electric mixer on medium-high speed, beat eggs until they fall in thick ribbons when beater is lifted, about 6 minutes. In three additions, fold eggs into the chocolate mixture. Pour batter into prepared pan and smooth top. Bake 30 to 35 minutes, or until a toothpick inserted near middle of cake comes out with a few moist crumbs and cake is evenly raised. Cool completely on a wire rack. To remove cake, run a knife around edge of pan. Dip bottom of pan into hot water for 1 minute, then turn cake out onto cutting board. Turn right side up onto a serving platter. Serve with whipped cream, if desired.